1. Field of the Invention
The invention relates to a process for producing diffractive light guide elements, to an apparatus for producing the diffractive light guide elements and to the light guide elements thus obtainable.
2. Discussion of Background Information
Light guide elements are also referred to as diffusers. Light guide elements and diffusers are understood to mean elements, especially films, with light-guiding properties. The simplest form of diffusers is that of diffusing screens which have a scattering action but do not permit directed guiding. However, the achievement of directed light guiding is of high significance for many optical applications, since many different effects can thus be achieved, for example an increase in the light yield in the case of displays, adjustment of the scatter and viewing angle of displays, side light-independent reflection for daylight projection, increase of contrast and others.
For the development of light guide properties, a refractive index gradient is generated. The principle of generating a refractive index gradient is known. As described, for example, in U.S. Pat. Nos. 5,552,261 and 5,529,473, the diffusion of monomers with a refractive index increased or else reduced relative to the surrounding liquid matrix can be utilized for the generation of a refractive index gradient. The “Colburn-Haines effect” known in photopolymers for directed diffusion with subsequent polymerization in the regions exposed in patterns leads to an increase in the density and hence to an increase or reduction in the refractive index. Thereafter, the refractive index gradient profile is fixed by whole surface curing (full-area curing). Instead of simple organic monomers, photosensitive components of inorganic nature have subsequently also been used, with which a greater difference in refractive index was achievable.
The production of such light guide elements is likewise known in principle, for instance by holographic processes. Light guide elements can be produced, for example, by embossing surface structures with light-guiding action or by pointwise exposure (for example with perforated masks) of photosensitive polymer films (photopolymers).
For example, WO 03/058292 describes processes for producing optical elements with gradient structure, in which nanoscale particles embedded in a solid matrix are obtained by generating a potential difference in a nanocomposite material composed of a curable matrix material with particles dispersed therein, such that directed diffusion of the nanoscale particles proceeds with formation of the concentration gradient, and the nanocomposite material having the concentration gradient cures. The nanocomposite material may be a photosensitive material.
DE-A-10200760 describes a process for producing a refractive index gradient film, in which a refractive index gradient is generated in a polymerizable, solid or gel-form nanocomposite material, for example by lithography or local exposure, and is then fixed by curing.
Patent applications PCT/EP2005/013685 and PCT/EP2005/013683 to the applicant describe processes for producing optical components with refractive index gradients, in which the refractive index gradient is generated in a hybrid material, said hybrid material comprising a soluble organic polymer and a mono- or polynuclear metal complex with photopolymerizable or thermally polymerizable ligands. Local irradiation or heating of the hybrid material forms the concentration gradient, which is then fixed by curing.
According to the above prior art, the light guide elements are produced by a two-stage process. In DE-A-10200760, for example, a photosensitive composition is applied to a backing by customary coating methods and dried. When polymer films are used as backings, the composite formed is provided with a cover film for protection and wound up for storage. In a second stage, the photosensitive composition applied is then structured, for which the cover film usually has to be removed.
The disadvantages of these two-stage processes arise from the necessity of drying only under very mild conditions after the coating, since the diffusion of the more highly refractive monomers or of the nanoparticles is very limited in more highly consolidated layers and the formation of a structure, such as a holographic structure, becomes virtually impossible in the subsequent pattern exposure process. This in turn causes a very great mechanical sensitivity of the layer applied and the tendency to stick to the mask. Both effects cause very high defect rates in the diffuser layers, which are not tolerable for many applications. An additional factor is that each process step, for example rolling up and unrolling again and accompanying transport processes, increases the error rate further. Moreover, current technology is restricted to film widths of no more than 60 cm. Specifically, the following disadvantages in particular arise in the state of the art:
1. The sandwich produced in the first process step is very soft and free-flowing and very sensitive to mechanical stress as a result of the process. As a result of the repeated handling, there is inevitably mechanical stress and these mechanical stresses can lead to minimal deformations in the surface, which are optically significantly amplified by the subsequent second step (exposure). This is caused by the fact that, in the course of exposure, so-called “light pipes” form along the direction of irradiation. Thus, if unevenness, such as small dents, bumps, bulges or scratches, is present in the sandwich to be exposed, which are referred to hereinafter as microdefects, the photosensitive material is irradiated at very different angles in the vicinity of these microdefects, which leads to macroscopically visible disruption of the parallelism of the light pipes and hence to macroscopically clearly visible disruption in the visual appearance and in the visual effect of the overall diffuser. Such diffusers are unsuitable in particular for use in the display sector.
2. The two-stage process requires storage times between the first and the second process step. During this storage, there may be aging processes in the light-sensitive material, which prevent reproducible diffuser production. Moreover, the storage of the sandwich material in the wound state, as a result of different pressures in the interior and exterior of the sandwich roll and as a result of the influence of gravity, leads to periodic occurrence of layer thickness variations which lead to clearly visible periodic variations in the optical effect of the diffuser film produced therefrom, since the light pipe structure comprises volume phase holograms whose effect (diffraction efficiency) depends greatly on the thickness. These too are macroscopic defects which are an obstacle to use of the diffusers, especially in the display sector.
3. For large-size applications of diffusers, for example in projection screens or large-size LCD-TV displays, it is necessary to join two or more diffuser films in longitudinal direction such that no seam is visible. This is because the width of the optics required for the exposure cannot be scaled up as desired for technical reasons and also for reasons of cost, for example cannot be scaled up to 1.60 m. The seamless joining of two or more diffusers produced in the above-described two-stage process by different processes known in the prior art is, though, not possible in the quality required for the abovementioned applications.
4. The two-stage process is very expensive owing to the numerous process steps.
5. When widths of above 50 to 60 cm are employed, owing to the fundamentally unavoidable inhomogeneities in relation to exposure and drying or temperature programming, warpage with corrugated or nonplanar edges occurs. These regions cannot be utilized for the above-described possible applications. Larger areas are possible only by joining smaller parts. Although this problem can be reduced to a certain degree by the mounting of oblique running wheels on the film edge, it is not possible overall according to the state of the art with the two-stage process to achieve greater widths.
It was therefore an object of the present invention to provide a process with which the above-described macroscopic defects of light guide elements can be avoided and which allows production of diffusers with high width which significantly exceeds the width of available exposure optics.
It has now been found that, surprisingly, this object is achieved by a process for producing diffractive light guide elements with a continuous roll-to-roll process.
The diffractive light guide elements comprise optical and microoptical light guide elements.